Soybeans are a major cash crop and investment commodity in Ncirth America and elsewhere. Soybean oil is one of the most widely used edible oils, arid soybeans are used throughout the world in both animal feed and in human food production.
Brown stem rot (BSR) is a fungal disease of soybean Glycine max (L.) Merr.! caused by Phialophora gregata (Allington and Chamberlain) W. Gams (syn. Cephalospodum gragatum Allington and Chamberlain). BSR occurs in many of the soybean production areas of the north-central USA (Allington 1946), the southeastern USA (Phillips 1970; Ross and Smith 1963), and Canada; and has been reported in Egypt, Japan, Mexico, and Yugoslavia (Sinclair and Backman 1989). It has caused an estimated average loss per year of 553,333 metric tons (approximately $112 million) of soybean production during each of the years 1989 to 1991 in the north-central USA (Doupnik, Jr. 1993). The severity of infection of soybean by P. gregata depends on the level of pathogenicity and the population density of the pathogen, as well as temperature, soil fertility, and the maturity status of the soybean variety (Gray 1971 and 1985; Mengistu and Gau 1986; Waller et al., 1992; Weber et al. 1966).
The environmental factors affecting BSR infection of soybean can be difficult for breeders to uniformly control with certainty. Susceptible plants not showing symptoms for BSR may be mistakenly classified as resistant because the environmental conditions were not conducive to the development of the disease. Thus, using genetic markers that are linked to BSR resistance loci as a way to screen soybean populations and select for linked BSR resistance loci may be more reliable than scoring BSR symptoms in field screenings.
BSR resistance in the soybean plant-introductions `PI 84.946-2` (Chamberlain and Bernard 1968), `PI 437.833` (Hanson et al. 1988), and `PI 437.970` (Willmot and Nickell 1989) is controlled by the dominant alleles Rbs.sub.1, Rbs.sub.2, and Rbs.sub.3 respectively (Hanson et al. 1988; Willmot and Nickell 1989). Other plant introductions having BSR resistance have also been identified (Nelson et al. 1989) but their respective resistance loci have not been determined. Most commercially available BSR-resistant soybean lines inherited their resistance from PI 84.946-2 through several cycles of breeding. Segregation among F.sub.2:3 families from the cross "Century" X PI 84.946-2 indicates that PI 84.946-2 has two unlinked BSR resistance loci (Sebastian and Nickell 1985). These two loci should be Rbs.sub.1 and Rbs.sub.3 because the BSR-resistant soybean lines `L78-4094` and `BSR101` have Rbs.sub.1 and Rbs.sub.3, respectively, and both lines are descendent from PI 84.946-2 as their only source of BSR resistance (Sebastian and Nickell 1985; Hanson et al. 1988; Tachibana et al. 1987; Eathington et al. 1995). Rbs.sub.3 is now assigned to BSR101 because all F.sub.2 plants and F.sub.2:3 families from the cross PI 437.970 X BSR101 were resistant to BSR (Eathington et al. 1995), and without segregation, the resistance locus is the same for both PI 437.970 and BSR101.
Genetic markers can be used to indirectly select for agronomically favorable genes among segregating individuals in plant breeding populations. Marker-assisted selection (MAS) is particularly useful when the desired trait is largely affected by the environment, which, as noted, often cannot be adequately controlled to optimize the expression of the trait. The greater the effect of an environment on a trait and the less that environment can be controlled, the less will be that trait's heritability and concomitant predictability of phenotypes. Genetic markers have highly reproducible phenotypes because their environment can be well controlled in the laboratory, conveying to them a heritability and predictability approaching unity. If a polymorphic genetic marker occurs within a few centimorgans of a gene affecting a desired trait, recombination events between the marker and gene would occur rarely, providing the high heritability and low error rate needed for reliable indirect selections for the favorable gene and the trait it governs.
Marker-assisted selection for BSR resistance can only be accomplished after one or more markers are found that are genetically linked to one or more BSR-resistance loci. For this reason RFLP markers to a locus associated with BSR resistance in the soybean variety BSR101 were genetically mapped. Based on the literature already described, this mapped resistance locus should be Rbs.sub.3. These markers, and other markers that may be found linked to these markers, provide a new and valuable tool to soybean breeders for selecting and developing future soybean cultivars having BSR resistance from Rbs.sub.3.